Hardware sprites on the FPGA computer

Sprite definition data consists of 16 lines, each line described by 16 pixels, each pixel defined by 4 bits: xrgb. This means that one sprite line consists of 8 bytes (two pixels per byte), so total bytes needed for the sprite definition is 8x16 bytes == 128 bytes.

  mov r0, sprite_def

  ; sprite definition

How this stuff works? First of all, I had to decide how to implement sprites. I have decided to fetch all sprite data during the vertical blanking interval (VBI). During VBI, the video subsystem starts fetching sprite data by reading the 8-byte sprite structure starting from the address of 56 decimal (the address and data bus are 16-bit, so the computer is word-oriented (reads two bytes at the same time), and the actual address is set to

56 >> 1 == 28):

In the next clock cycle, the system is in the READ_SPRITES state. The first thing that we do in the READ_SPRITES state is fetching the sprite definition address which is present at the data bus, since we have initiated a memory read from within the previous state.

Then we need to prepare the address bus for the next state in which we will fetch the x coordinate of the sprite. We do that by setting the address bus to (58 + (sprite_counter << 3)) for all sprites, having the sprite_counter iterating from 0 to 15:

In the READ_SPRITE_X state, we fetch the x coordinate of the sprite which was ready at the data bus, and then we prepare to read the y coordinate in the next state:

In the READ_SPRITE_Y state, we fetch the y coordinate of the sprite which was ready at the data bus, and then we prepare to read the sprite transparent color in the next state:

In the READ_SPRITE_TRANSPARENT_COLOR state, we fetch the transparent color of the sprite, and then put the address of the sprite definition to the address bus so we can fetch it in the next state:

In the READ_SPRITE_DATA state we start reading sprite definition from the memory. We do it for each line of the sprite (16 lines per sprite), and within the line, for each word containing four pixels of the sprite line definition.

When we finish loading all sprite definition data for the current sprite, then we do the same for other sprite until we read all sprite definition data. Then we then set the address bus to load the pixel data at the (0, 0) position on the screen, and move to the V_BLANK state:

V_BLANK: begin
  pixels <= data;
  state <= SCAN_IDLE;
  rd <= 1'bz;
  wr <= 1'bz;
  mem_read <= 1'b0;
end
In the V_BLANK state we read the pixels of the frame buffer at the (0, 0) coordinate, and then set the all the control signals to high impedance and set the state to SCAN_IDLE. We will leave the SCAN_IDLE state when the the time comes to start displaying pixels starting from the (0, 0) coordinate.

Displaying sprite data

During the scanline processing, we need to display both original pixels from the frame buffer as well as the sprite data, and we need to make sure that the original pixels must be displayed through the transparent sprite color.

This is done in the following code:

if (valid) begin
  for (i = 0; i < SPRITE_NUM; i = i+1) begin
    if ((sprite_addr[i] != 16'b0) &&
       (xx >= sprite_x[i]) &&
       (xx < (sprite_x[i] + 16)) &&
       (yy >= sprite_y[i]) &&
       (yy < (sprite_y[i] + 16))) begin
      sprite_found = 1'b1;
      if (
        sprite_pixels[i][yy - sprite_y[i]][60-(((xx - sprite_x[i]) << 2) ) + 0] != sprite_transparent_color[i][0] ||
        sprite_pixels[i][yy - sprite_y[i]][60-(((xx - sprite_x[i]) << 2) ) + 1] != sprite_transparent_color[i][1] ||
        sprite_pixels[i][yy - sprite_y[i]][60-(((xx - sprite_x[i]) << 2) ) + 2] != sprite_transparent_color[i][2]
      ) begin
        r <= sprite_pixels[i][yy - sprite_y[i]][60-(((xx - sprite_x[i]) << 2) ) + 0] == 1'b1;
        g <= sprite_pixels[i][yy - sprite_y[i]][60-(((xx - sprite_x[i]) << 2) ) + 1] == 1'b1;
        b <= sprite_pixels[i][yy - sprite_y[i]][60-(((xx - sprite_x[i]) << 2) ) + 2] == 1'b1;
      end 
      else begin
        r <= pixels[12 - ((xx & 3) << 2) + 0] == 1'b1;
        g <= pixels[12 - ((xx & 3) << 2) + 1] == 1'b1;
        b <= pixels[12 - ((xx & 3) << 2) + 2] == 1'b1;
      end
    end 
  end
  if (!sprite_found) begin
    r <= pixels[12 - ((xx & 3) << 2) + 0] == 1'b1;
    g <= pixels[12 - ((xx & 3) << 2) + 1] == 1'b1;
    b <= pixels[12 - ((xx & 3) << 2) + 2] == 1'b1;
  end
  else begin
    sprite_found = 1'b0;
  end
end
else begin
  // blanking -> no pixels
  r <= 1'b0;
  g <= 1'b0;
  b <= 1'b0;
end
end

Conclusion